1. Field of the Invention
The invention relates to a steering control apparatus of a vehicle that includes variable steering ratio controlling means for changing a relationship between the steering angle of steered wheels and the steering angle of the steering by applying a driving force to the steering input.
2. Description of the Related Art
A steering control apparatus (VGRS: Variable Gear Ratio Steering) that includes variable steering ratio controlling means that can change the ratio between the steering angle of a steering wheel and the steering angle of steered wheels is known (for example, refer to Japanese Patent Application Publication No. JP-A-H11-321684). This technology aims at realizing desired steering response under various operational conditions of the vehicle by, for example, changing the steering ratio according to the vehicle speed. To realize such variable steering ratio controlling means, for example, a transmission portion such as a gear is provided at a connection point between the input shaft in the steering wheel side and the output shaft in the tie rods side, and the relative rotation angles of the input shaft and the output shaft are changed by applying driving force to the transmission portion by an actuator. It should be noted that the actuator that applies the driving force to the transmission portion may constitute the transmission portion. Note that for such steering ratio control there is a method in which feedback control is performed so as to achieve a target steering angle.
In feedback control for variable steering ratio control as described above, the accuracy of position control is improved by setting the control gain high. However, when the driver is not holding the steering wheel (will be referred to as a “non-holding state” where necessary), the reaction force from the actuator is not received by the steering wheel side, and therefore the control system becomes unstable. As a result, the steering wheel may vibrate. Such a non-holding state may occur while the steering wheel is being returned to the neutral position.
In the technology disclosed in Japanese Patent Publication Application No. JP-A-H11-321684, for the purpose of reducing vibration when the driver is not holding the steering wheel, such a non-holding state is detected based on the torque that the driver applies to the steering wheel, and when it is detected, the control gain is reduced.
However, when the control gain is reduced in this manner, the accuracy of position control and the steering response are deteriorated, and thus there is the problem that the positional deviation due to external disturbances becomes large.